The answer to your question is - there is more than one kind of illusion.
Glamers and figments change your senses, so they can't hold up weight. Characters trying to stand on them will fall through (and immediately disbelieve, since they have proof the illusion was a fake).
A figment spell creates a false sensation...
A glamer spell changes a subject’s sensory qualities...
Because figments and glamers are unreal, they cannot produce real effects the way that other types of illusions can.
Shadows can hold up weight just fine.
A shadow spell creates something that is partially real from extradimensional energy. Such illusions can have real effects.
However, some shadow spells can be partially disbelieved, in which case there is a % chance that the object doesn't "work" and the creature falls through. See shadow conjuration:
Shadow objects or substances have normal effects except against those who disbelieve them. Against disbelievers, they are 20% likely to work.
Patterns and phantasms affect the mind directly. These illusions can make someone believe they are standing on the floor, while in reality they have fallen down a pit.
Like a figment, a pattern spell creates an image that others can see, but a pattern also affects the minds of those who see it or are caught in it.
A phantasm spell creates a mental image that usually only the caster and the subject (or subjects) of the spell can perceive. This impression is totally in the minds of the subjects.
For any non-instantaneous spell, the effect disappears once the duration ends. Regardless of what kind of spell was creating the bridge (illusion, conjuration, evocation, what have you) the creature standing on it will fall in the hole.